1. Field of the Invention
This invention relates to an oxide film removing apparatus and to a method for removing an oxide film or the like formed on a surface of a silicon wafer, a surface of a polysilicon film and a surface of an amorphous silicon film (these surfaces are referred to as "silicon layer surfaces" hereinafter) during manufacturing of a semiconductor device, and particularly, it relates to an apparatus and a method for removing the oxide film or the like by using hydrogen fluoride gas.
2. Description of the Related Art
A natural oxide film (SiO.sub.2) formed on, for example, a surface of a silicon wafer in a manufacturing process of the semiconductor device adversely affects the operation characteristics of the device. SiO.sub.2 should be, therefore, removed from the surface of the wafer before formation of a film by CVD, sputtering or the like.
A method of removing the silicon oxide film from a surface of a silicon wafer is disclosed in Japanese Patent Publication No. 62-502930 (referred to as "the first prior art method") corresponding to PCT/US86/01714 based on U.S. application Ser. No. 770,27. According to the method, an anhydrous hydrogen fluoride gas, vapor and a dry nitrogen gas are fed from separate sources with their flow rates controlled to obtain a predetermined mixture ratio. The mixture is fed to a reactive chamber in which gas substrate is stored. The exposure of the substrate surface to these gases removes the silicon oxide film.
Another method is disclosed in, for example, SUBMICRON ULSI PROCESS TECHNIOUE, Proceedings of The Seventh Super LSI Ultracleaning Technology Symposium 1988-7, published by Realize Inc., pp 173-194, (hereinafter referred to as "the second prior art method"). A super high concentration anhydrous hydrogen fluoride gas containing an extremely small amount of water is fed to a reactive chamber with a high density nitrogen gas of super low water content as a carrier gas. SiO.sub.2 on a silicon wafer and hydrogen fluoride are reacted with each other in the reactive chamber. As a result, the silicon oxide film is removed.
According to the first prior art method, the gases of the respective components having their flow rates separately controlled are mixed with each other. The composition of vapor mixture to be supplied to the substrate is individually controlled. As a result, the concentration of hydrogen fluoride in the anhydrous hydrogen fluoride gas source is as high as 99.99%. Extremely high concentration liquid anhydrous hydrogen fluoride is used also in the second prior art method.
The foregoing will be described with reference to the drawings. As shown in FIG. 1, a high concentration hydrogen fluoride gas produced in a chemical factory equipped with safety facilities is conventionally brought to a semiconductor manufacturing facility. This hydrogen fluoride gas is used for removing an oxide film as the semiconductor manufacturing facility.
As described above, a semiconductor manufacturing facility conventionally necessitates a very high concentration anhydrous hydrogen fluoride gas which is extremely poisonous and harmful to humans. The anhydrous hydrogen fluoride gas must not therefore be allowed to leak into air. Maintenance and control of the gas system and a changing of gas cylinders always requires the greatest possible care. Consequently, a facility for removing the oxide film must be equipped with the same safety features as those in a chemical factory treating harmful chemical materials which is highly disadvantageous.
Furthermore, vapor is added to the anhydrous hydrogen fluoride gas in the first prior art method. The application of this method to etching of a silicon oxide film on a silicon wafer, for example, causes such an undesirable side reaction as follows: EQU SiO.sub.2 +2H.sub.2 O=S.sub.i (OH).sub.4 ( 1)
or such a reverse reaction to the etching occurs to cause colloidal metasilicate (H.sub.2 SiO.sub.3) or silicon dioxide (SiO.sub.2) to be attached on the wafer surface, which results in contamination. EQU 2SiF.sub.4 +2H.sub.2 O.fwdarw.SiO.sub.2 +SiF.sub.6.sup.2- +2H.sup.+ +2HF (2)
In the second prior art method, the water content of the supply gas should be maintained at an extremely low value in order to reproduce the process. This consumes much time in controlling the water content in the gas supply system.
According to the first prior art method, the three constituent gases have their flow rates controlled separately to control the composition of the vapor mixture, which makes the arrangement of the entire control system very complicated. The arrangement of the gas supply system itself is also complicated because the vapor mixture is obtained by mixing the gases of the three components after the flow rates thereof are controlled.